Interferometric heterodyne optical encoder system

ABSTRACT

An encoder interferometry system includes an interferometer positioned to receive first and second beams having different frequencies, in which the interferometer has at least one polarizing beam splitting element for directing the first beam along a measurement path to define a measurement beam and the second beam along a reference path to define a reference beam. The encoder interferometry system further includes a encoder scale positioned to diffract the measurement beam at least once, a detector positioned to receive the measurement and reference beams after the measurement beam diffracts from the encoder scale, and an output component positioned to receive the measurement and reference beams before they reach the detector and deflect spurious portions of the first and second beam away from the detector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Application No.61/438,394, filed on Feb. 1, 2011 and entitled, “INTERFEROMETRICHETERODYNE OPTICAL ENCODER SYSTEM,” the contents of which are herebyincorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to interferometric heterodyne optical encodersystems and methods to implement a heterodyne measurement of phasechanges occurring in reflected or transmitted beams diffracted from anencoder scale.

BACKGROUND

In some cases, interferometric measuring systems monitor changes in therelative position of a measurement object based on an opticalinterference signal. For example, an interferometer generates theoptical interference signal by overlapping and interfering a“measurement beam” that interacts with (e.g., reflects from) themeasurement object with a second beam, sometimes called a “referencebeam.” Changes in the relative position of the measurement objectcorrespond to changes in the phase of the measured optical interferencesignal.

However, many interferometric measuring systems include nonlinearitiessuch as what are known as “cyclic errors.” In general, cyclic errors areunderstood as measurement errors that are periodic with the relativeposition of a measurement object. The cyclic errors can be expressed ascontributions to the phase and/or the intensity of the measuredinterference signal and have a sinusoidal dependence on the change in anoptical path difference between the measurement beam and the secondbeam. The cyclic errors can be produced by “beam mixing,” in which aportion of an input beam that nominally forms the second beam propagatesalong a measurement path and/or a portion of an input beam intended topropagate along the measurement path instead propagates along areference path. Such beam mixing can be caused by ellipticity in thepolarizations of the input beam and/or imperfections in theinterferometer components. Cyclic errors can also be produced byimperfections in components such as retro-reflectors and/or phaseretardation plates that produce undesired ellipticities in beams in theinterferometer. If not compensated, the foregoing cyclic errors canlimit the accuracy of position changes measured by the interferometersystem.

SUMMARY

This disclosure relates to interferometric heterodyne optical encodersystems and methods to implement a measurement of phase changesoccurring in reflected or transmitted beams diffracted from an encoderscale. In a preferred embodiment, an encoder system includes an outputcomponent positioned to deflect spurious beams away from a detector, andthus reduce the magnitude of cyclic errors produced by ghost beamsand/or polarization mixing effects.

In certain aspects, the disclosure features an encoder system includingan interferometer positioned to receive first and second beams havingdifferent frequencies, in which the interferometer has at least onepolarizing beam splitting element for directing the first beam along ameasurement path to define a measurement beam and the second beam alonga reference path to define a reference beam. The encoder system furtherincludes an encoder scale positioned to diffract the measurement beam atleast once, a detector positioned to receive the measurement andreference beams after the measurement beam diffracts from the encoderscale, and an output component positioned to receive the measurement andreference beams before they reach the detector and deflect spuriousportions of the first and second beam away from the detector. Thespurious portions include a portion of the first beam directed along thereference path and a portion of the second beam directed along themeasurement path because of imperfections in any of a polarization ofthe first beam, a polarization of the second beam, and the polarizingbeam splitting element.

Implementations of the encoder system can include one or more of thefollowing features and/or features of other aspects. For example, theoutput component can be a birefringent output component. The outputbirefringent component can include a prism pair. A first prism in theprism pair can include a birefringent wedge and a second prism in theprism pair can include a glass wedge.

In some implementations, the system further includes a linear polarizer,an output fiber-optic lens, and output fiber to couple the measurementand reference beams from the output component to the detector. The inputfiber-optic lens and the input component can be combined to cause theangular difference in propagation direction between the first and secondbeams to be between about 0.1 and 10 mrad or between about 0.5 and 5mrad.

In some implementations, the system further includes a source configuredto generate the first and second beams having the different frequencies,wherein the source is further configured to cause the first and secondbeam to have substantially orthogonal polarizations. The source caninclude an acousto-optic modulator, an electro-optic modulator, or aZeeman-split laser to generate the different frequencies. The source caninclude an input component to introduce an angular difference inpropagation direction between the first and second beams. The source canbe configured to cause both the first and second beams to have a linearpolarization, a circular polarization or an elliptical polarization.

The input component can be a birefringent input component. The inputcomponent can include a prism pair. The source can further include apair of input polarization-preserving fibers for carrying the first andsecond beams toward the interferometer, and an input fiber-optic lensfor coupling the first and second beams from the input fibers to theinput component. The input and output components can correspond todifferent portions of a common birefringent component.

In some implementations, the output component is further configured tocombine the measurement and reference beams with one another.

In some implementations, the detector is configured to measure aninterferometric intensity signal based on interference between themeasurement beam and the reference beam.

In some implementations, the encoder scale comprises a one-dimensionalgrating.

In some implementations, the interferometer includes a measurementretroreflector, in which the encoder scale diffracts the measurementbeam to the measurement retroreflector, receives the measurement beamback from the measurement retroreflector, and then diffracts themeasurement beam back to the polarizing beam splitting element. Theinterferometer can have a polarizing beam splitter that includes thepolarizing beam splitting element. The interferometer can furtherinclude a reference retroreflector for retroreflecting the referencebeam back to the polarizing beam splitting element.

In some implementations, the first and second beams define a first setof input beams, in which the interferometer is further positioned toreceive a second set of input beams to define a second measurement beamand a second reference beam. The encoder scale can be positioned todiffract the second measurement beam at least once, and the system canfurther include a second detector positioned to receive the secondmeasurement and reference beams after the second measurement beamdiffracts from the encoder scale. The system can further include asecond output component positioned to receive the second measurement andreference beams before they reach the second detector and deflectspurious portions of the second set of input beams away from the seconddetector. The interferometer can further include a second measurementretroreflector, wherein the encoder scale diffracts the secondmeasurement beam to the second measurement retroreflector, receives thesecond measurement beam back from the second measurement retroreflector,and then diffracts the second measurement beam back to the polarizingbeam splitting element. The second detector can be configured to measurea second interferometric intensity signal based on interference betweenthe second measurement beam and the second reference beam. The systemcan further include a signal processing system to determine changes inthe position of the encoder scale along at least two degrees of freedombased on the first and second interferometric intensity signals.

In some implementations, the first and second beams have an angulardifference in propagation prior to impinging on the polarizing beamsplitting element. For example, the angular difference in propagationcan be between about 0.1 and 10 mrad or the angular difference inpropagation can be between about 0.5 and 5 mrad.

In some implementations, the beam splitting element includes a beamsplitting interface. The beam splitting element can be a prism cube, adiffractive optical element, or a birefringent element.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features and advantages willbe apparent from the description, the drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of an interferometricheterodyne optical encoder system.

FIG. 2 is a schematic diagram of an embodiment of an interferometricheterodyne optical encoder.

FIG. 3 is a schematic diagram of an embodiment of an interferometricheterodyne optical encoder system.

FIG. 4 is a schematic diagram illustrating an expanded portion of theencoder system of FIG. 3.

FIG. 5A is an example image of an interference pattern obtained from aninterferometric heterodyne optical encoder system.

FIG. 5B is a graph of an integrated intensity value of the interferencepattern of FIG. 5A.

FIG. 5C is an example image of an interference pattern obtained from aninterferometric heterodyne optical encoder system.

FIG. 5D is a graph of an integrated intensity value of the interferencepattern of FIG. 5C.

FIG. 6 is a graph of the integrated signal strength versus number offringes recorded by a detector.

FIG. 7 is a schematic diagram of an embodiment of an interferometricheterodyne optical encoder system.

FIG. 8 is a schematic diagram of an embodiment of an interferometricheterodyne optical encoder system.

FIG. 9 is a schematic diagram of a beam delivery portion of the systemof FIG. 8.

FIG. 10 is a schematic diagram of a fiber output portion of the systemof FIG. 8.

FIG. 11 is a schematic diagram of an embodiment of a lithography toolthat includes an interferometer.

FIG. 12A and FIG. 12B are flow charts that describe steps for makingintegrated circuits.

DETAILED DESCRIPTION

The present disclosure is directed towards error compensation ininterferometric heterodyne optical encoder systems. The disclosure belowis organized into three sections. A first section of the disclosure,entitled “Interferometric Heterodyne Optical Encoder Systems,” relatesto a general description of how an interferometric heterodyne opticalencoder system may operate. A second section of the disclosure, entitled“Angled-Beam Error Correction,” relates to structures and methods forcorrecting errors in interferometric heterodyne optical encoder systems.A third section of the disclosure entitled, “Lithography ToolApplications,” relates to structures and methods for employing theinterferometric heterodyne optical encoder system in a lithographysystem.

Interferometric Heterodyne Optical Encoder Systems

Referring to FIG. 1, an example of an interferometric heterodyne opticalencoder system 100 includes a light source module 120 (e.g., including alaser), an optical assembly 110, a measurement object 101, a detectormodule 130 (e.g., including a polarizer and a detector), and anelectronic processor 150. Generally, light source module 120 includes alight source and can also include other components such as beam shapingoptics (e.g., light collimating optics), light guiding components (e.g.,fiber optic waveguides) and/or polarization management optics (e.g.,polarizers and/or wave plates). Various embodiments of optical assembly110 are described below. The optical assembly is also referred to as the“encoder head.” A Cartesian coordinate system is shown for reference.

Measurement object 101 is positioned some nominal distance from opticalassembly 110 along the Z-axis. In many applications, such as where theencoder system is used to monitor the position of a wafer stage orreticle stage in a lithography tool, measurement object 101 is movedrelative to the optical assembly in the X- and/or Y-directions whileremaining nominally a constant distance from the optical assemblyrelative to the Z-axis. This constant distance can be relatively small(e.g., a few centimeters or less). However, in such applications, thelocation of measurement object typically will vary a small amount fromthe nominally constant distance and the relative orientation of themeasurement object within the Cartesian coordinate system can vary bysmall amounts too. During operation, encoder system 100 monitors one ormore of these degrees of freedom of measurement object 101 with respectto optical assembly 110, including a position of measurement object 101with respect to the x-axis, and further including, in certainembodiments, a position of the measurement object 101 with respect tothe y-axis and/or z-axis and/or with respect to pitch and yaw angularorientations. In some embodiments, the optical assembly can be movedwith respect to the measurement object 101, whereas in some otherembodiments, both the measurement object 101 and the optical assemblyare moveable.

To monitor the position of measurement object 101, source module 120directs an input beam 122 to optical assembly 110. Optical assembly 110derives a measurement beam 112 from input beam 122 and directsmeasurement beam 112 to measurement object 101. Optical assembly 110also derives a reference beam (not shown) from input beam 122 anddirects the reference beam along a path different from the measurementbeam. For example, optical assembly 110 can include a beam splitter thatsplits input beam 122 into measurement beam 112 and the reference beam.The measurement and reference beams can have orthogonal polarizations(e.g., orthogonal linear polarizations).

Measurement object 101 includes an encoder scale 105, which is ameasuring graduation that diffracts the measurement beam from theencoder head into one or more diffracted orders. In general, encoderscales can include a variety of different diffractive structures such asgratings or holographic diffractive structures. Examples or gratingsinclude sinusoidal, rectangular, or saw-tooth gratings. Gratings can becharacterized by a periodic structure having a constant pitch, but alsoby more complex periodic structures (e.g., chirped gratings). Ingeneral, the encoder scale can diffract the measurement beam into morethan one plane. For example, the encoder scale can be a two-dimensionalgrating that diffracts the measurement beam into diffracted orders inthe X-Z and Y-Z planes. The encoder scale extends in the X-Y plane overdistances that correspond to the range of the motion of measurementobject 110.

In the present embodiment, encoder scale 105 is a grating having gratinglines that extend orthogonal to the plane of the page, parallel to theY-axis of the Cartesian coordinate system shown in FIG. 1. The gratinglines are periodic along the X-axis. Encoder scale 105 has a gratingplane corresponding to the X-Y plane and the encoder scale diffractsmeasurement beam 112 into one or more diffracted orders in the Y-Zplane.

At least one of these diffracted orders of the measurement beam (labeledbeam 114), returns to optical assembly 110, where it is combined withthe reference beam to form an output beam 132. For example, theonce-diffracted measurement beam 114 can be the first-order diffractedbeam.

Output beam 132 includes phase information related to the optical pathlength difference between the measurement beam and the reference beam.Optical assembly 110 directs output beam 132 to detector module 130 thatdetects the output beam and sends a signal to electronic processor 150in response to the detected output beam. Electronic processor 150receives and analyzes the signal and determines information about one ormore degrees of freedom of measurement object 101 relative to opticalassembly 110.

In certain embodiments, the measurement and reference beams have a smalldifference in frequency (e.g., a difference in the kHz to MHz range) toproduce an interferometry signal of interest at a frequency generallycorresponding to this frequency difference. This frequency ishereinafter referred to interchangeably as the “heterodyne” frequency orthe “reference” frequency, and is denoted as ω_(R) (with respect toangular frequency). Information about the changes in the relativeposition of the measurement object generally corresponds to a phase ofthe interferometry signal at this heterodyne frequency. Signalprocessing techniques can be used to extract this phase. In general, themoveable measurement object causes this phase term to be time-varying.In this regard, the first order time derivative of the measurementobject movement causes the frequency of the interferometry signal toshift from the heterodyne frequency by an amount referred to herein asthe “Doppler” shift.

The different frequencies of the measurement and reference beams can beproduced, for example, by laser Zeeman splitting, by acousto-opticalmodulation, using two different laser modes, or internal to the laserusing birefringent elements, among other techniques. The orthogonalpolarizations allow a polarizing beam-splitter to direct the measurementand reference beams along different paths, and combine them to form theoutput beam that subsequently passes through a polarizer, which mixesthe orthogonally polarized components so they can interfere. In theabsence of target motion the interference signal oscillates at theheterodyne frequency, which is just the difference in the opticalfrequencies of the two components. In the presence of motion theheterodyne frequency incurs a change related to the velocity of thetarget through well-known Doppler relations. Accordingly, monitoringchanges in the heterodyne frequency allows one to monitor motion of thetarget relative to the optical assembly.

In the embodiments described below, the “input beam” generally refers tothe beam emitted by the light source module. For heterodyne detection,the input beam can include components having slightly differentfrequencies, as discussed above.

While encoder scale 105 is depicted in FIG. 1 as a structure that isperiodic in one direction, more generally, the measurement object caninclude a variety of different diffractive structures that appropriatelydiffract the measurement beam. In some embodiments, the measurementobject can include a diffractive structure (e.g., a encoder scale) thatis periodic in two directions (e.g., along the x- and y-axis),diffracting the measurement beam into beams in two orthogonal planes. Ingeneral, the diffractive structure of the encoder scale and sourcemodule are selected so that the encoder system provides one or morediffracted measurement beams having sufficient intensity to establishone or more detectable interference signals when combined withcorresponding reference beams, within the geometrical constraints forthe system. In some embodiments, the source module provides an inputbeam having a wavelength in a range from 400 nm to 1,500 nm. Forexample, the input beam can have a wavelength of about 633 nm or about980 nm. Note that, in general, the frequency splitting of the heterodynesource results in only a very small difference between the wavelength ofthe two components of the input beam, so even though the input beam isnot strictly monochromatic it remains practical to characterize theinput beam by a single wavelength. In some embodiments, the sourcemodule can include a HeNe laser, a laser diode or other solid-statelaser source, a light-emitting diode, or a thermal source such as ahalogen light with or without a filter to modify the spectral bandwidth.

In general, the diffractive structure (e.g., grating pitch) can varydepending on the wavelength of the input beam and the arrangement ofoptical assembly and diffracted orders used for the measurement. In someembodiments, the diffractive structure is a grating having a pitch in arange from about 1λ to about 20λ, where λ is a wavelength of the source.The grating can have a pitch in a range from about 1 μm to about 10 μm.

Various embodiments of encoder systems are possible. For example, themeasurement beam can be incident on measurement object 101 at anincident angle such that the diffracted measurement beam does notsatisfy the Littrow condition. The Littrow condition refers to anorientation of a diffractive structure, such as a grating, with respectto an incident beam where the diffractive structure directs thediffracted beam back towards the source such that the diffracted beam isco-linear with the incident beam. When the diffracted beam is notco-linear with the incident beam, the Littrow condition is notsatisfied.

In some embodiments, the encoder system is arranged so that themeasurement beam makes a double pass to the encoder scale and a twicediffracted order of the measurement beam is used for the measurement.For example, referring to FIG. 2, an interferometric heterodyne opticalencoder system 200 includes an optical assembly 210 having a referenceretroreflector 212, a measurement retroreflector 214, and a polarizingbeam-splitter 216. The system 200 also contains a target 201, a lasersource 218, a detector module 240 including a detector 242 and polarizer244, and an electronic processor 250. A Cartesian coordinate system isshown for reference.

To perform the monitoring, source 218 directs an input beam 270 topolarizing beam-splitter 216. Polarizing beam-splitter 216 derives ameasurement beam 274 and a reference beam 275 from input beam 270, wherethe polarizations of the measurement beam 274 and reference beam 275 areorthogonal. As shown in the example of FIG. 2, measurement beam 274 ispolarized orthogonal to the plane of the figure (s-polarization), whilesecondary beam 275 is polarized in the plane of the figure(p-polarization). However, the measurement beam and reference beam canhave any polarization (e.g., linear, circular, or elliptical) as long asthey are distinguishable from one another (e.g., orthogonallypolarized).

The beam-splitter 216 directs the measurement beam 274 to target 201.Target 201 can include an encoder scale 205 that diffracts the incidentmeasurement beam 274, providing a once-diffracted measurement beam thatcorresponds to a non-zeroth diffracted order (e.g., first order orsecond order) of measurement beam 274. The diffracted measurement beam274 then is redirected by measurement retroreflector 212 to impinge onthe encoder scale 205 once more to provide a twice-diffractedmeasurement beam. The twice-diffracted measurement beam 274 then returnsto polarizing beam-splitter 216. The polarizing beam-splitter 216 thencombines the twice-diffracted measurement beam 274 with the referencebeam 275 to form an output beam 290, where the reference beam 275 hasbeen redirected by the reference retroreflector 212. Although FIG. 2shows a polarizing beam-splitter, other optical components may be usedthat also direct beams based on polarization properties. These opticalcomponents include, for example, prism cubes, diffractive optics,birefringent components and reflective surfaces (bare or coated) onwhich beams are incident at glancing angles.

The output beam 290 includes phase information related to the opticalpath difference between the component corresponding to thetwice-diffracted measurement beam and the component corresponding to thereference beam. Polarizing beam-splitter 216 then directs output beam290 to detector module 240. At detector module 240, polarizer 244 mixesthe measurement and reference beam components of the output beam 290before the output beam is incident on detector 242. This can beachieved, for example, by orienting the transmission axis of polarizer244 so that it transmits a component of s-polarized light and acomponent of p-polarized light (e.g., by orienting the transmission axisat 45° with respect to the plane of the page). Upon detecting the mixedcomponents of the output beam 290, the detector 242 of detector module240 subsequently sends a signal to electronic processor 250 in response.

Electronic processor 250 receives and analyses the signal and determinesinformation about one or more degrees of freedom of target 201 relativeto the optical assembly 210. Specifically, the electronic processordetermines this information based in part on a heterodyne phase of thesignal. Accordingly, monitoring changes in the heterodyne frequencyallows one to monitor motion of the target 201 relative to the opticalassembly 210.

As shown in FIG. 2, the polarizing beam-splitter causes the separationof the measurement beam and reference beam components from the inputbeam. In some implementations, however, the separation of themeasurement beam and the reference beam components may be imperfect,e.g., a portion of the measurement beam component does not follow themeasurement beam path and/or a portion of the reference beam componentdoes not follow the reference beam path, leading to beam “mixing.”

In general, the spurious beams that mix with other desired beams arecalled “ghost beams.” The ghost beams may have different amplitudes,different phase offsets, and/or different frequencies from the beamswith which they combine, resulting in a shift in a detected interferencesignal frequency or phase, or a change in detected interference signalamplitude. As a result, cyclic errors in measurements of the position ofthe encoder scale can occur, in which the errors are periodic with theposition of the encoder scale relative to the optical assembly.

Ghost beams can be caused by various imperfections in theinterferometric encoder system. For example, unintended ellipticity inthe polarizations of the different frequency components of the inputbeam may lead to leakage through a polarizing beam-splitter that is usedto split the input beam along respective measurement and referencepaths. That is to say, a portion of the measurement beam having a firstpolarization and frequency may exit the beam splitter along thereference path instead of the measurement path as intended, whereas aportion of the reference beam having a second different polarization andfrequency may exit the beam splitter along the measurement path insteadof the reference path as intended. In some embodiments, unintendedelliptical polarization in the input beam is due to polarization mixinginherent in the illumination source. Leakage through a polarizingbeam-splitter also may be caused by imperfections in the beam-splitter,itself. For example, in some embodiments, the polarizing beam-splittermay have a low extinction ratio, where the extinction ratio is thetransmission of an unwanted beam component relative to the beam wantedcomponent. In some embodiments, leakage through a polarizing beamsplitter is due to misalignment of the input beam with the beamsplitter's plane of polarization.

Ghost beams also can arise due to unwanted reflections from othercomponents of the interferometric encoder system. For example, in someembodiments the interferometric encoder system employs one or morequarter wave plates. If the quarter wave plates do not enable 100% beamtransmission, a portion of the beam incident on the wave plate may beunintentionally reflected into a measurement path or reference path.

In some embodiments, reflections from the encoder scale also lead toghost beams. For example, a portion of the diffracted measurement beammay unintentionally propagate toward the beam-splitter along a path thatis co-linear with the incident measurement beam (i.e., beforediffraction from the encoder scale). It should be noted that theforegoing examples of sources of cyclic errors are not exhaustive andthat other mechanisms for generating such errors also exist.

Interference of the desired beams and the ghost beams, whether fromleakage or other imperfections in the interferometer system, can lead tovarious types of cyclic errors that cause deviations in the detectedoutput beam. Electronic or algorithmic means can be applied to mitigatethe cyclic errors that are produced as a result of the systemimperfections. However, in some instances, the errors are too large,leading to significant uncertainty in the measurement results.

Angled-Beam Error Correction

In some embodiments, cyclic errors can be minimized by increasing aseparation angle between the measurement and reference beams. Forexample, FIG. 3 is a schematic diagram of a heterodyne optical encodersystem 300 for suppressing non-linearities and cyclic errors, such asthose caused by beam mixing. The basic operating principles of theencoder system 300 are similar to those of system 100 shown in FIG. 1.However, the encoder system 300 includes one or more polarization opticsthat introduce and remove a difference in propagation angle betweenmeasurement and reference beams to enhance separation between the beams.This allows the beams to be distinguished by their propagation angle inaddition to their polarization, thus reducing the magnitude of cyclicerrors produced by ghost beams and polarization mixing effects.

Similar to the interferometric encoder system of FIG. 2, system 300includes a detector module 340 having a detector 342 and polarizer 344,a source 318, an electronic processor 350, a target 301 that includes anencoder scale 305, and an optical assembly 310 having a referenceretroreflector 312, a measurement retroreflector 314, and a polarizingbeam-splitter 316. The optical assembly 310 also includes a polarizationoptic 320, such as a birefringent component.

During operation of the system 300, laser source 318 produces an inputbeam 370 having co-linear, orthogonally polarized components defining aprimary beam portion 371 and a secondary beam portion 372. For example,the primary beam portion can be linearly polarized out of the plane ofthe figure (e.g., s-polarization), whereas the secondary beam portioncan be linearly polarized in the plane of the figure (e.g.,p-polarization). The orthogonally polarized portions can further includea heterodyne frequency shift between them. It should be noted that theprimary beam portion and secondary beam portions can have anypolarization (e.g., linear, circular, or elliptical) as long as they aredistinguishable from one another (e.g., orthogonally polarized).

The input beam 370 is incident on the first polarization optic 320. Thepolarization optic 320 imparts a small angle of separation a between thedirections of propagation of the primary and secondary beam portions371, 372. The amount of angular separation can be determined based onthe initial beam polarization of the input beam 370, which may bedefined by the laser source 318, as well as the material and orientationof the polarization optic 320, to be described further below. Therespective polarization states of the primary and secondary beamcomponents are generally preserved upon emerging from the polarizationoptic 320.

The primary beam 371 and secondary beam 372 then are further separatedaccording to their polarizations by polarizing beam-splitter 316 todefine measurement beam 374 and reference beam 375, respectively. Apolarizing beam-splitter interface 311 redirects the measurement beam374 along a measurement path toward a encoder scale 305 whilesimultaneously transmitting the reference beam 375 along a referencepath toward reference retroreflector 312. The measurement beam 374 isdiffracted into one or more diffracted orders by the encoder scale 305located on target 301. In preferred embodiments, the diffractedmeasurement beam then propagates along a direction away from polarizingbeam-splitter 316 toward measurement retroreflector 314 where thediffracted beams is redirected back to the encoder scale 305. Themeasurement beam 374 then diffracts again from the encoder scale 305 andreturns to the polarizing beam-splitter 310. At the same time, thereference beam 375 is redirected by reference retroreflector 312 backtowards the polarizing beam-splitter 316.

The twice-diffracted measurement beam 374 then is reflected by thepolarizing beam-splitter interface 311 while the reference beam 375 istransmitted through the interface 311. Due to the angular separation ofthe input beam 370 initially imparted by the first polarization optic320, however, the twice-diffracted measurement beam 374 and thereference beam 375 do not travel along parallel paths toward thedetector 342. Instead, the beam paths of the twice-diffractedmeasurement beam 374 and reference beam 375 exiting the beam-splitter316 propagate with an angle of separation a between the beam paths.

The converging beams are incident on a second polarization optic 321(e.g., a birefringent component) that removes the angle of separationbetween the twice-diffracted measurement beam path and the referencebeam path. As a result, the measurement beam 374 and reference beam 375transmit through the second polarization optic 321 and propagateco-linearly as an output beam. Polarizer 344 mixes the co-linear beams.The mixed beams are detected by detector 342, which produces aninterference signal. The detector 342 subsequently sends theinterference signal to electronic processor 350.

However, beams without the proper polarization and/or the properpropagation angle including, for example, ghost beams will not betransmitted co-linearly through the second polarization optic 321.Instead, those spurious beams are deflected by the second polarizationoptic 321 away from the detector.

Electronic processor 350 receives and analyzes the heterodyne frequencyof the interference signal to determine information about one or moredegrees of freedom of target 301 relative to the encoder system 300.Similar to the encoder system 200 shown in FIG. 2, changes in theheterodyne frequency correspond to changes in the velocity of the target301 through well-known Doppler relations. In contrast to the encodersystem 200, however, the non-parallelism of spurious beams exiting thesecond polarization optic 321 suppresses the contribution of those beamsto the interferometric measurement. Accordingly, in someimplementations, the motion of the target 301 relative to the encodersystem 300 can be monitored and cyclic errors due to beam mixing can bereduced.

In the example of FIG. 3, each of the first polarization optic 320 andthe second polarization optic 321 can include a birefringent componentsuch as a birefringent prism pair. For example, a first prism in thepair can include a birefringent wedge prism and a second prism in thepair can include an isotropic wedge prism. In some embodiments, thebirefringent wedge and isotropic wedge can be fused together (e.g.,using an optical adhesive) to form a single composite birefringentcomponent. The wedge materials and orientations can be selected so thata first beam having a selected polarization transmits through thebirefringent component without any angular deflection, whereas a secondbeam having a polarization orthogonal to the first beam is angularlydeflected by the birefringent component. For example, the isotropicprism may be formed from any of the common optical glasses, such as BK7or fused silica. The birefringent prism may be formed of one or moredifferent materials, such as crystalline quartz SiO2, rutile TiO2,sapphire Al2O3, lithium niobate LiNbO3, calcite CaCO3. The prism pairmay also be of two different birefringent materials with differentcrystalline orientation or degrees of birefringence. Although thebirefringent components shown in FIG. 3 each include a prism pair, thecomponent can include a single prism, three prisms or some othercombination of prisms.

The angular separation imparted by the polarization optic 320 can bebetween about 0.05 mrad and 20 mrad, including for example, betweenabout 0.1 and 10 mrad, or between about 0.5 and 5 mrad.

In some embodiments, it is not necessary to use two separatepolarization optics. Instead, a single polarization optic, such as asingle birefringent prism pair, can be used to impart the angularseparation to the orthogonally polarized portions of the input beam andalso remove the angular separation from the diffracted measurement beamand reference beam that exit the polarizing beam-splitter.

FIG. 4 is a schematic diagram illustrating an expanded portion of theencoder system 300 of FIG. 3 during operation of the system 300. Inparticular, FIG. 4 shows the measurement beam 374 and reference beam 375as they exit the polarizing beam-splitter 316 and propagate towardsdetector 342. In the absence of beam-mixing or ghost beams, themeasurement beam 374 (which has, for example, s-polarization) and thereference beam (which has, for example, p-polarization) approach thebirefringent component 321 at an angle of convergence equal to the angleof separation α. The birefringent component 321 then removes the angleof separation so that both beams propagate co-linearly toward polarizer344 and detector 342.

In contrast, if the beams incident on the birefringent component 321 areghost beams (e.g., beams that inadvertently transmit through or reflectfrom the polarizing beam-splitter due to an imperfect beam-splitterinterface or due to imperfect polarization), that propagate towards thebirefringent component 321 with an improper angle, such ghost beams aredeflected upon passing through the birefringent component 321 andpropagate away from the detector as spurious polarized beams. Forexample, referring to FIG. 3, if an incorrectly polarized beampropagates parallel to beam 374, the incorrectly polarized beam willexit the birefringent component 321 as a spurious p-polarized beam.Similarly, if an incorrectly polarized beam propagates parallel to beam375, the incorrectly polarized beam will exit the birefringent component321 as a spurious s-polarized beam. The angle between the two spuriousbeams is approximately twice the angle of separation α.

Although the spurious beams may still pass through the polarizer 344 andonto the detector 342 even after deflection by the component 321, thespurious beams do not significantly alter the primary interferencesignal produced by the measurement beam and reference beam. Instead, theoptical interference signal produced by the spurious beams averages awaywhen integrated over the spatial extent of the detector given that thespurious components diverge from the compensated output beam 390 andfrom each other. For example, the spurious beams may generate a fringepattern on the detector surface that, when integrated across thedetector surface, results in a weak contamination signal. If desired,the spurious beams may also be removed by spatial filtering (not shownin FIG. 3). In contrast, when the measurement beam and reference beamhaving the proper propagation angle and polarization transmitco-linearly to the detector after passing through the output component321, the heterodyne interference signal produced by the beams has astrong contrast (i.e., ratio of signal modulation to average signalvalue).

FIG. 5A is an example image of a simulated interference pattern based onthe interference between two beams having a non-zero separation anglebetween them (e.g., non-co-linear spurious beams). The simulation wasproduced using MATHCAD® 14 simulation software. FIG. 5B is a graphdepicting the integrated intensity value of the interference pattern ofFIG. 5A integrated over the detector surface versus time. In contrast,FIG. 5C is an example image of a simulated interference pattern based onthe interference between two beams having no angle of separation betweenthem (e.g., co-linear measurement and reference beams). The beams aremodeled as uniform in intensity. In practice, the beams typically willhave a more Gaussian intensity profile. FIG. 5D is a graph depicting theintegrated intensity value of the interference pattern of FIG. 5Cintegrated over the detector surface versus time. As shown in theexamples of FIG. 5, integration of the interference pattern from thespurious beams corresponds to a signal with relatively low magnitudewhereas the magnitude of the integrated interference signal of themeasurement and reference beams is much greater.

To determine the degree to which unwanted spurious signals areattenuated, the total number of fringes v across the detector area isdetermined. In an example, it is assumed that the detector area iscircular, such that the number of fringes is given by the followingequation:

$\begin{matrix}{{v = {\frac{4R}{\lambda}\sin \; (\alpha)}},} & (1)\end{matrix}$

where R is the detector radius, λ is the wavelength, α is the divergenceangle between the interfering beams. Next, the heterodyne signal, whichis the real part of the complex representation, is calculated. Forsimplicity, it is assumed that the beam profile is perfectly flat acrossthe detector aperture (though in practice the beam profile may vary),such that a complex representation of the heterodyne signal can beexpressed as:

I(t,v,x)=exp [i(ft+πvx/R)],  (2)

where f is the heterodyne frequency, x is a linear coordinate orthogonalto the interference fringes, and the signal strength has been normalizedto 1. The total signal is the integration over the entire detector areais provided by the equation

$\begin{matrix}{{I\left( {t,v} \right)} = {\frac{1}{area}{\int_{area}{{I\left( {t,v,x} \right)}{{a}.}}}}} & (3)\end{matrix}$

In polar coordinates (r, θ), Eq. 3 becomes

$\begin{matrix}{{{I\left( {t,v} \right)} = {\frac{1}{\pi \; R^{2}}{\int_{0}^{R}{\int_{0}^{\pi}{{I\left\lbrack {t,v,{x\left( {r,\theta} \right)}} \right\rbrack}{\theta}\; r{r}}}}}},} & (4)\end{matrix}$

where

x(r,θ)=r cos(θ).  (5)

Incorporating Eq. 1, Eq. 5 then becomes

$\begin{matrix}{{I\left( {t,v} \right)} = {\left\{ {\frac{2}{\pi \; R^{2}}{\int_{0}^{R}{\int_{0}^{\pi}{{\exp \left\lbrack {{\left( {r\; \pi \; {v/R}} \right)}{\cos (\theta)}} \right\rbrack}\ {\theta}\; r{r}}}}} \right\} {{\exp \left( {\; f\; t} \right)}.}}} & (6)\end{matrix}$

Using

$\begin{matrix}{{J_{0}\left( u^{\prime} \right)} = {\frac{1}{\pi}{\int_{0}^{\pi}{{\exp \left\lbrack {\; u^{\prime}{\cos (\theta)}} \right\rbrack}\ {\theta}}}}} & (7) \\{{{uJ}_{1}(u)} = {\int_{0}^{u}{u^{\prime}{J_{0}\left( u^{\prime} \right)}\ {u^{\prime}}}}} & (8)\end{matrix}$

with

u′=(rπv/R)  (9)

u=πv,  (10)

the heterodyne signal can be expressed as:

$\begin{matrix}{{I\left( {t,v} \right)} = {\left\lbrack \frac{2{J_{1}\left( {\pi \; v} \right)}}{\pi \; v} \right\rbrack {{\exp \left( {\; f\; t} \right)}.}}} & (11)\end{matrix}$

The integrated signal strength may therefore be proportional to

$\begin{matrix}{{S(v)} = {{\frac{2{J_{1}\left( {\pi \; v} \right)}}{R}}.}} & (12)\end{matrix}$

A graph of the simulated integrated signal strength versus number offringes recorded across a detector diameter is shown in FIG. 6. As shownin FIG. 6, it is evident that beams having a small relative divergence,and thus low number of fringes (see Eq. 1) will contribute strongly tothe heterodyne signal. As an example, if the detector area is a circleof radius R=0.5 mm and the beam profile is uniform over this area, thenaccording to Eq. (1), a divergence angle as small as α=1 mrad at awavelength λ=633 nm will generate 3.16 fringes across the field of view.The resulting contribution to the signal is only 0.0125 compared to amaximum of 1 for co-linear beams. Although the exact quantitativecalculation may be different for different beam profiles (e.g., Gaussianbeam profiles), the basic principle set forth above applies.

Other configurations of the interferometric heterodyne optical encodersystem are also possible. For example, in some embodiments, theinterferometric heterodyne optical encoder system incorporates twoseparate detector modules, each of which separately interferes anddetects different diffracted orders with a reference beam to bothimprove motion sensitivity resolution and distinguish between encodermotion along a primary beam axis and a secondary axis.

For example, FIG. 7 is a schematic diagram of an encoder system 700 thatemploys two separate detectors 740, 741. In the example of FIG. 7, lasersource 718 produces an input beam 770 having co-linear, orthogonallypolarized components. The input beam 770 is incident on a non-polarizingbeam splitter 760, where the beam 770 is separated into separate inputbeams 770 a (solid line in FIGS. 7) and 770 b (dotted line in FIG. 7).The non-polarizing beam-splitter 760 can also include a retroreflector762 arranged to redirect at least one of the beams 770 a, 770 b so bothbeams propagate co-linearly toward the polarizing beam-splitter 716.

Prior to reaching beam-splitter 716 each input beam passes through acorresponding polarization optic that imparts an angular separation tothe orthogonally polarized portions of the beams. As shown in theexample of FIG. 7, polarization optic 720 a imparts a first angle ofseparation α₁ (not shown) to the s- and p-polarized portions of inputbeam 770 a to derive a first measurement beam 774 a and first referencebeam 775 a. Similarly, polarization optic 720 b imparts a second angleof separation α₂ (not shown) to the s- and p-polarized portions of inputbeam 770 b to derive a second measurement beam 774 b and secondreference beam 775 b.

Both measurement beams are subsequently re-directed by the beam-splitterinterface 711 toward a target 701 having an encoder scale 705. In thepresent example, encoder scale 705 diffracts each of the incidentmeasurement beams 774 a, 774 b into one or more diffracted orders. Atleast one of the diffracted orders from measurement beam 774 a isredirected by first measurement retroreflector 714 a back to the encoderscale 705. Similarly, at least one of the diffracted orders from thesecond measurement beam 774 b is redirected by second measurementretroreflector 714 b back to the encoder scale 705. Both diffractedorders then are diffracted again back toward the polarizingbeam-splitter 716. In addition, both reference beams are transmittedthrough the beam-splitter interface 711 toward the referenceretroreflector 712 where they are redirected back to the beam-splitter716. Upon exiting beam-splitter 716, twice-diffracted measurement beam774 a and reference beam 775 a converge towards polarization optic 721 awith an angle of separation α₁ (not shown). Similarly, twice-diffractedmeasurement beam 774 b and reference beam 775 b converge towardspolarization optic 721 b with an angle of separation α₂ (not shown).

Each polarization optic subsequently removes the angle of separationbetween the corresponding twice-diffracted measurement beam/referencebeam pair to provide either a first output beam 790 a or second outputbeam 790 b. Polarizer 744 then mixes the co-linear and orthogonalportions of output beam 790 a to produce a first interference patternand mixes the co-linear and orthogonal portions of 790 b to produce asecond interference pattern, in which the first interference pattern andthe second interference pattern are detected by detector 740 anddetector 741 respectively. The detectors 740, 741 then each produce aninterference signal that is sent to the processor 750. Although a singlepolarizer 744 is shown in the example of FIG. 7, a separate polarizercan be provided for each output beam 790 a and 790 b.

Spurious beams without the proper polarization and/or the properpropagation angle, however, will not be transmitted co-linearly throughthe polarization optics 721 a, 721 b. Instead, those spurious beams aredeflected by the polarization optics 721 a, 721 b away from thedetectors.

Electronic processor 750 receives and analyzes the heterodyne frequencyof the interference signals. Since motion of the encoder scale in the Zdirection is common to the measurements of both detector modules, whilemotion of the encoder scale along the X direction is detected withopposite signs, the motion along the X and Z directions can bedistinguished by a composite signal consisting of the sum or differenceof the two separate phases for each interference signal. In someembodiments, additional detector modules can be provided for measurementof the displacements along the Y-axis. For such 2-dimensional (2D)applications (X and Y measurements) an area grating can be used. Forexample, encoder scale 705 can be periodic in both the X- andY-directions.

As in other embodiments, each of the first polarization optics, 720 aand 720 b, can include a birefringent component, such as a birefringentprism pair, where a first prism in the pair is a birefringent wedge anda second prism in the pair is an isotropic wedge. The birefringent wedgeand isotropic wedge can be separate components or fused together (e.g.,using an optical adhesive) to form a single composite birefringentcomponent. The wedge materials and orientations can be selected so thata first beam having a selected polarization transmits through thebirefringent component without any angular deflection, whereas a secondbeam having a polarization orthogonal to the first beam is angularlydeflected by the birefringent component. In some embodiments, a singlepolarization optic, such as a single birefringent prism pair, can beused to impart the angle of separation to the orthogonally polarizedportions of each input beam and also remove the angles of separationfrom the diffracted measurement beams and reference beams that exit thepolarizing beam-splitter.

In some embodiments, optical fibers are used in place of the free-spacelight source input. For example, FIG. 8 is a schematic diagram of anexemplary interferometric heterodyne optical encoder system 800 thatincludes a dual fiber input 818 and a fiber output 808. In the example,the fiber input 818 is a polarization preserving fiber that includes afirst fiber core and a second separate fiber core. The first fiber coreis arranged to provide a first beam having a first polarization whereasthe second fiber core is arranged to provide a second beam having asecond orthogonal polarization.

The polarization optic 820 (e.g., a birefringent prism pair having abirefringent prism A and glass prism B) operates in combination with afocusing lens 819 to impart an angle of separation to the orthogonallypolarized input beams. In contrast, the polarization optic 821 (e.g., asecond birefringent prism pair) operates to remove an angle ofseparation between a diffracted measurement beam and reference beam, andto deflect spurious beams away from fiber output 808. The lens 809 canbe used to couple an interference signal into the fiber output 808,where the interference signal is obtained by mixing the diffractedmeasurement beam and reference beam at polarizer 844. Other componentsof encoder system 800 are similar to those described above with respectto system 300 and/or system 700.

FIG. 9 is a schematic diagram that shows the beam delivery portion ofthe example system 800 in more detail. As shown in the example of FIG.9, lens 819 is incorporated at the end of the input fiber pigtail 817.The polarization optic 820 includes a birefringent prism 822 and a glassprism 823. The two fiber cores of the dual fiber input 818 can beseparated by various distances including, but not limited to betweenabout 75 μm to 175 μm, between about 100 μm to 150 μm or between about125 μm to 135 μm. Lens 819 can have various focal lengths depending onthe collimated beam widths desired. For example, the focal length oflens 819 can be approximately 7.0 mm for a beam having a width of 1.25mm at 1/e² of the maximum collimated beam intensity, resulting in aninitial angular separation of the measurement beam from the referencebeam of about 1.023° to 1.105°. Other angles of separation can beimparted to the beams. For example, the input fiber optic lens 819 andthe input polarization optic 820 can combine to cause the angulardifference in propagation direction between the first and second beamsto be between about 0.1 and 10 mrad, or between about 0.5 and 5 mrad.

FIG. 10 is a schematic diagram that shows the fiber output portion ofFIG. 8 including polarization optic 821 and polarizer 844. As shown inthe example, lens 809 is incorporated at the end of the output fiberpigtail 807. In some implementations, the polarization optic 821, whichcan include a birefringent prism and isotropic prism, reduces theangular separation between a diffracted measurement beam and a referencebeam to 1 mrad or 2 fringes across the 1/e² beam diameter. In someimplementations, the foregoing reduction is sufficient to substantiallyreduce sources of cyclic error.

In general, any of the analysis methods described above, includingdetermining information about a degree of freedom of the encoder scales,can be implemented in computer hardware or software, or a combination ofboth. For example, in some embodiments, the electronic processors can beinstalled in a computer and connected to one or more encoder systems andconfigured to perform analysis of signals from the encoder systems.Analysis can be implemented in computer programs using standardprogramming techniques following the methods described herein. Programcode is applied to input data (e.g., interferometric phase information)to perform the functions described herein and generate outputinformation (e.g., degree of freedom information). The outputinformation is applied to one or more output devices such as a displaymonitor. Each program may be implemented in a high level procedural orobject oriented programming language to communicate with a computersystem. However, the programs can be implemented in assembly or machinelanguage, if desired. In any case, the language can be a compiled orinterpreted language. Moreover, the program can run on dedicatedintegrated circuits preprogrammed for that purpose.

Each such computer program is preferably stored on a storage medium ordevice (e.g., ROM or magnetic diskette) readable by a general or specialpurpose programmable computer, for configuring and operating thecomputer when the storage media or device is read by the computer toperform the procedures described herein. The computer program can alsoreside in cache or main memory during program execution. The analysismethods can also be implemented as a computer-readable storage medium,configured with a computer program, where the storage medium soconfigured causes a computer to operate in a specific and predefinedmanner to perform the functions described herein.

Lithography Tool Applications

Lithography tools are especially useful in lithography applications usedin fabricating large scale integrated circuits such as computer chipsand the like. Lithography is the key technology driver for thesemiconductor manufacturing industry. Overlay improvement is one of thefive most difficult challenges down to and below 22 nm line widths(design rules), see, for example, the International Technology Roadmapfor Semiconductors, pp. 58-59 (2009). See also M. S. Hibbs, “Systemoverview of optical steppers and scanners,” Microlithography, K. Suzuki,B. Smith, Eds., pp. 46-47 (CRC Press, Boca Raton, 2007).

Overlay depends directly on the performance, i.e., accuracy andprecision, of the metrology system used to position the wafer andreticle (or mask) stages. Since a lithography tool may produce $50-100M/year of product, the economic value from improved metrology systems issubstantial. Each 1% increase in yield of the lithography tool resultsin approximately $1 M/year economic benefit to the integrated circuitmanufacturer and substantial competitive advantage to the lithographytool vendor.

The function of a lithography tool is to direct spatially patternedradiation onto a photoresist-coated wafer. The process involvesdetermining which location of the wafer is to receive the radiation(alignment) and applying the radiation to the photoresist at thatlocation (exposure).

During exposure, a radiation source illuminates a patterned reticle,which scatters the radiation to produce the spatially patternedradiation. The reticle is also referred to as a mask, and these termsare used interchangeably below. In the case of reduction lithography, areduction lens collects the scattered radiation and forms a reducedimage of the reticle pattern. Alternatively, in the case of proximityprinting, the scattered radiation propagates a small distance (typicallyon the order of microns) before contacting the wafer to produce a 1:1image of the reticle pattern. The radiation initiates photo-chemicalprocesses in the resist that convert the radiation pattern into a latentimage within the resist.

To properly position the wafer, the wafer includes alignment marks onthe wafer that can be measured by dedicated sensors. The measuredpositions of the alignment marks define the location of the wafer withinthe tool. This information, along with a specification of the desiredpatterning of the wafer surface, guides the alignment of the waferrelative to the spatially patterned radiation. Based on suchinformation, a translatable stage supporting the photoresist-coatedwafer moves the wafer such that the radiation will expose the correctlocation of the wafer. In certain lithography tools, e.g., lithographyscanners, the mask is also positioned on a translatable stage that ismoved in concert with the wafer during exposure.

Encoder systems, such as those discussed previously, are importantcomponents of the positioning mechanisms that control the position ofthe wafer and reticle, and register the reticle image on the wafer. Ifsuch encoder systems include the features described above, the accuracyof distances measured by the systems can be increased and/or maintainedover longer periods without offline maintenance, resulting in higherthroughput due to increased yields and less tool downtime.

In general, the lithography tool, also referred to as an exposuresystem, typically includes an illumination system and a waferpositioning system. The illumination system includes a radiation sourcefor providing radiation such as ultraviolet, visible, x-ray, electron,or ion radiation, and a reticle or mask for imparting the pattern to theradiation, thereby generating the spatially patterned radiation. Inaddition, for the case of reduction lithography, the illumination systemcan include a lens assembly for imaging the spatially patternedradiation onto the wafer. The imaged radiation exposes resist coatedonto the wafer. The illumination system also includes a mask stage forsupporting the mask and a positioning system for adjusting the positionof the mask stage relative to the radiation directed through the mask.The wafer positioning system includes a wafer stage for supporting thewafer and a positioning system for adjusting the position of the waferstage relative to the imaged radiation. Fabrication of integratedcircuits can include multiple exposing steps. For a general reference onlithography, see, for example, J. R. Sheats and B. W. Smith, inMicrolithography: Science and Technology (Marcel Dekker, Inc., New York,1998), the contents of which is incorporated herein by reference.

Encoder systems described above can be used to precisely measure thepositions of each of the wafer stage and mask stage relative to othercomponents of the exposure system, such as the lens assembly, radiationsource, or support structure. In such cases, the encoder system'soptical assembly can be attached to a stationary structure and theencoder scale attached to a movable element such as one of the mask andwafer stages. Alternatively, the situation can be reversed, with theoptical assembly attached to a movable object and the encoder scaleattached to a stationary object.

More generally, such encoder systems can be used to measure the positionof any one component of the exposure system relative to any othercomponent of the exposure system, in which the optical assembly isattached to, or supported by, one of the components and the encoderscale is attached, or is supported by the other of the components.

An example of a lithography tool 1100 using an interferometric encodersystem 1126 is shown in FIG. 11. The interferometric encoder system isused to precisely measure the position of a wafer (not shown) within anexposure system. Here, stage 1122 is used to position and support thewafer relative to an exposure station. Scanner 1100 includes a frame1102, which carries other support structures and various componentscarried on those structures. An exposure base 1104 has mounted on top ofit a lens housing 1106 atop of which is mounted a reticle or mask stage1116, which is used to support a reticle or mask. A positioning systemfor positioning the mask relative to the exposure station is indicatedschematically by element 1117. Positioning system 1117 can include,e.g., piezoelectric transducer elements and corresponding controlelectronics. Although, it is not included in this described embodiment,one or more of the encoder systems described above can also be used toprecisely measure the position of the mask stage as well as othermoveable elements whose position must be accurately monitored inprocesses for fabricating lithographic structures (see supra Sheats andSmith Microlithography: Science and Technology).

Suspended below exposure base 1104 is a support base 1113 that carrieswafer stage 1122. Stage 1122 includes a measurement object 1128 fordiffracting a measurement beam 1154 directed to the stage by opticalassembly 1126. A positioning system for positioning stage 1122 relativeto optical assembly 1126 is indicated schematically by element 1119.Positioning system 1119 can include, e.g., piezoelectric transducerelements and corresponding control electronics. The measurement objectdiffracts the measurement beam back to the optical assembly, which ismounted on exposure base 1104. The interferometric encoder system can beany of the embodiments described previously.

During operation, a radiation beam 1110, e.g., an ultraviolet (UV) beamfrom a UV laser (not shown), passes through a beam shaping opticsassembly 1112 and travels downward after reflecting from mirror 1114.Thereafter, the radiation beam passes through a mask (not shown) carriedby mask stage 1116. The mask (not shown) is imaged onto a wafer (notshown) on wafer stage 1122 via a lens assembly 1108 carried in a lenshousing 1106. Base 1104 and the various components supported by it areisolated from environmental vibrations by a damping system depicted byspring 1120.

In some embodiments, one or more of the encoder systems describedpreviously can be used to measure displacement along multiple axes andangles associated for example with, but not limited to, the wafer andreticle (or mask) stages. Also, rather than a UV laser beam, other beamscan be used to expose the wafer including, e.g., x-ray beams, electronbeams, ion beams, and visible optical beams.

In certain embodiments, the optical assembly 1126 can be positioned tomeasure changes in the position of reticle (or mask) stage 1116 or othermovable components of the scanner system. Finally, the encoder systemscan be used in a similar fashion with lithography systems involvingsteppers, in addition to, or rather than, scanners.

As is well known in the art, lithography is a critical part ofmanufacturing methods for making semiconducting devices. For example,U.S. Pat. No. 5,483,343 outlines steps for such manufacturing methods.These steps are described below with reference to FIGS. 12A and 12B.FIG. 12A is a flow chart of the sequence of manufacturing asemiconductor device such as a semiconductor chip (e.g., IC or LSI), aliquid crystal panel or a CCD. Step 1251 is a design process fordesigning the circuit of a semiconductor device. Step 1252 is a processfor manufacturing a mask on the basis of the circuit pattern design.Step 1253 is a process for manufacturing a wafer by using a materialsuch as silicon.

Step 1254 is a wafer process that is called a pre-process wherein, byusing the so prepared mask and wafer, circuits are formed on the waferthrough lithography. To form circuits on the wafer that correspond withsufficient spatial resolution those patterns on the mask,interferometric positioning of the lithography tool relative the waferis necessary. The interferometry methods and systems described hereincan be especially useful to improve the effectiveness of the lithographyused in the wafer process.

Step 1255 is an assembling step, which is called a post-process whereinthe wafer processed by step 1254 is formed into semiconductor chips.This step includes assembling (dicing and bonding) and packaging (chipsealing). Step 1256 is an inspection step wherein operability check,durability check and so on of the semiconductor devices produced by step1255 are carried out. With these processes, semiconductor devices arefinished and they are shipped (step 1257).

FIG. 12B is a flow chart showing details of the wafer process. Step 1261is an oxidation process for oxidizing the surface of a wafer. Step 1262is a CVD process for forming an insulating film on the wafer surface.Step 1263 is an electrode forming process for forming electrodes on thewafer by vapor deposition. Step 1264 is an ion implanting process forimplanting ions to the wafer. Step 1265 is a resist process for applyinga resist (photosensitive material) to the wafer. Step 1266 is anexposure process for printing, by exposure (i.e., lithography), thecircuit pattern of the mask on the wafer through the exposure apparatusdescribed above. Once again, as described above, the use of theinterferometry systems and methods described herein improve the accuracyand resolution of such lithography steps.

Step 1267 is a developing process for developing the exposed wafer. Step1268 is an etching process for removing portions other than thedeveloped resist image. Step 1269 is a resist separation process forseparating the resist material remaining on the wafer after beingsubjected to the etching process. By repeating these processes, circuitpatterns are formed and superimposed on the wafer.

The encoder systems described above can also be used in otherapplications in which the relative position of an object needs to bemeasured precisely. For example, in applications in which a write beamsuch as a laser, x-ray, ion, or electron beam, marks a pattern onto asubstrate as either the substrate or beam moves, the encoder systems canbe used to measure the relative movement between the substrate and writebeam.

Other embodiments are within the scope of the following claims.

1. An encoder interferometry system comprising: an interferometerpositioned to receive first and second beams having differentfrequencies, the interferometer comprising at least one polarizing beamsplitting element for directing the first beam along a measurement pathto define a measurement beam and the second beam along a reference pathto define a reference beam; an encoder scale positioned to diffract themeasurement beam at least once; a detector positioned to receive themeasurement and reference beams after the measurement beam diffractsfrom the encoder scale; and an output component positioned to receivethe measurement and reference beams before they reach the detector anddeflect spurious portions of the first and second beam away from thedetector, the spurious portions comprising a portion of the first beamdirected along the reference path and a portion of the second beamdirected along the measurement path because of imperfections in any of apolarization of the first beam, a polarization of the second beam, andthe polarizing beam splitting element.
 2. The system of claim 1, whereinthe output component is a birefringent output component.
 3. The systemof claim 2, wherein the output birefringent component comprises a prismpair.
 4. The system of claim 3, wherein a first prism in the prism pairis a birefringent wedge and wherein a second prism in the prism pair isa glass wedge.
 5. The system of claim 1, further comprising: a linearpolarizer; an output fiber-optic lens; and output fiber to couple themeasurement and reference beams from the output component to thedetector.
 6. The system of claim 1, further comprising a sourceconfigured to generate the first and second beams having the differentfrequencies, wherein the source is further configured to cause the firstand second beam to have substantially orthogonal polarizations.
 7. Thesystem of claim 6, wherein the source comprises an acousto-opticmodulator, an electro-optic modulator, or a Zeeman-split laser togenerate the different frequencies.
 8. The system of claim 6, whereinthe source comprises an input component to introduce an angulardifference in propagation direction between the first and second beams.9. The system of claim 8, wherein the input component is a birefringentinput component.
 10. The system of claim 8, wherein the input componentcomprises a prism pair.
 11. The system of claim 8, wherein the sourcefurther comprises: a pair of input polarization-preserving fibers forcarrying the first and second beams toward the interferometer; and aninput fiber-optic lens for coupling the first and second beams from theinput fibers to the input component.
 12. The system of claim 8, whereinthe input and output components correspond to different portions of acommon birefringent component.
 13. The system of claim 1, wherein theoutput component is further configured to combine the measurement andreference beams with one another.
 14. The system of claim 1, wherein thedetector is configured to measure an interferometric intensity signalbased on interference between the measurement beam and the referencebeam.
 15. The system of claim 1, wherein the encoder scale comprises aone-dimensional grating.
 16. The system of claim 1, wherein theinterferometer comprises a measurement retroreflector, and wherein theencoder scale diffracts the measurement beam to the measurementretroreflector, receives the measurement beam back from the measurementretroreflector, and then diffracts the measurement beam back to thepolarizing beam splitting element.
 17. The system of claim 16, whereinthe beam splitting element is a polarizing beam splitter and wherein theinterferometer further comprises a reference retroreflector forretroreflecting the reference beam back to the polarizing beam splitter.18. The system of claim 1, wherein the first and second beams define afirst set of input beams and wherein the interferometer is furtherpositioned to receive a second set of input beams to define a secondmeasurement beam and a second reference beam, wherein the encoder scaleis positioned to diffract the second measurement beam at least once, andwherein the system further comprises a second detector positioned toreceive the second measurement and reference beams after the secondmeasurement beam diffracts from the encoder scale.
 19. The system ofclaim 18, further comprising a second output component positioned toreceive the second measurement and reference beams before they reach thesecond detector and deflect spurious portions of the second set of inputbeams away from the second detector.
 20. The system of claim 18, whereinthe interferometer further comprises a second measurementretroreflector, wherein the encoder scale diffracts the secondmeasurement beam to the second measurement retroreflector, receives thesecond measurement beam back from the second measurement retroreflector,and then diffracts the second measurement beam back to the polarizingbeam splitting element.
 21. The system of claim 18, wherein the seconddetector is configured to measure a second interferometric intensitysignal based on interference between the second measurement beam and thesecond reference beam.
 22. The system of claim 21, further comprising asignal processing system to determine changes in the position of theencoder scale along at least two degrees of freedom based on the firstand second interferometric intensity signals.
 23. The system of claim 1,wherein the first and second beams have an angular difference inpropagation prior to impinging on the polarizing beam splitting element.24. The system of claim 23, wherein the angular difference inpropagation is between about 0.1 and 10 mrad.
 25. The system of claim24, wherein the angular difference in propagation is between about 0.5and 5 mrad.
 26. The system of claim 11, wherein the input fiber opticlens and the input component combine to cause the angular difference inpropagation direction between the first and second beams to be betweenabout 0.1 and 10 mrad.
 27. The system of claim 26, wherein the angulardifference is between about 0.5 and 5 mrad.
 28. The system of claim 1,wherein the beam splitting element comprises a beam splitting interface.29. The system of claim 1, wherein the beam splitting element is a prismcube.
 30. The system of claim 1, wherein the beam splitting elementcomprises a diffractive optical element.
 31. The system of claim 1,wherein the beam splitting element comprises a birefringent element. 32.The system of claim 6, wherein the source is configured to cause boththe first and second beams to have a linear polarization, a circularpolarization or an elliptical polarization.